The 'plebgate' row that engulfed Andrew Mitchell was sparked by a police officer who posed as a member of the public and falsely claimed to have witnessed the events, it was said tonight.

The officer allegedly wrote to his local MP giving details of the Chief Whip's behaviour when he was blocked from cycling out of the main gates in Downing Street.

Number 10 said the claims - which emerged after a member of the diplomatic protection squad was arrested - were "extremely serious".

"Any allegations that a serving police officer posed as a member of the public and fabricated evidence against a Cabinet Minister are exceptionally serious," a spokesman said.

"It is therefore essential that the police get to the bottom of this as a matter of urgency."

Mitchell has always denying using the word 'pleb'

However, Metropolitan Police Commissioner Bernard Hogan-Howe has insisted he has seen nothing to undermine the official account of officers who were there.

The claims came as CCTV images of the former Chief Whip's exchange with police were revealed by a Channel 4 Dispatches investigation, raising questions over whether the encounter was as angry as leaked police logs suggested.

Andrew Mitchell was accused of calling police “f***ing Plebs”, who should learn their "f***ing place” after officers on duty in Downing Street refused to open the gate so that the Tory MP could stay on his bicycle.

Although there is no sound, the MP - who clung on to his job for a month before finally resigning - can be seen with his bicycle talking to three officers by the main gate for around 20 seconds.

He then wheels it over to the side gate and exits through it. Footage from other cameras suggest that there were few members of the public passing by at the time - apparently contradicting the leaked police log.

CCTV footage of the incident has been released

However in the first interview since his resignation, Mitchell told Political Correspondent Michael Crick the "toxic phrases" attributed to him were completely untrue and used to destroy my political career and were used to toxify the Conservative Party.

He told Channel 4: "I never said phrases like that at all, I would never call someone an F’ing pleb, anyone who knows me well would know that it is absolutely not in me to use phrases like that.

“I got a call from Downing Street to say that The Sun were going to splash on an altercation with the police and my first reaction was there’s not really much of an altercation to splash on and it was only later that I heard these toxic phrases which had been attributed to me.

"I always knew that the emails were false, although extremely convincing," he added. "It has shaken my lifelong support and confidence in the police.

"I believe now there should be a full inquiry so we can get to the bottom of this."

The Sun has stood by its original story despite the latest revelations.

Andrew Mitchell on his bicycle

Mitchell was forced to resign in the midst of what rapidly became known as "plebgate" with the police federation, Labour leaders and even members of his own party criticising his behaviour.

The printed police report, leaked to the Telegraph, only served to further condemn Mitchell.

According to the leaked logs there were “several members of the public present - around the Gate - they heard the altercation and were “visibly shocked”.

The very next day the Deputy Chief Whip John Randall received an email from one of his constituents saying that he and his nephew had both seen and heard the incident and - his version matched the police story that later appeared in the Telegraph– almost word for word.

The email was sent from Ruislip in North West London the night after the incident in Downing Street to Randall. It said: ““Other people/tourists standing with us were also shocked and some were even, inadvertently filming the incident …”

However the new stills show there aren’t any crowds of people watching and listening. Additionally, the Times have reported that a serving police officer posed as a member of the public to report the incident to Randall, who according to the Guardian had a "strained" relationship with Mitchell.

That officer was arrested at the weekend as part of an Independent Police Complaints Commission (IPCC) investigation.

Of the letter, Mitchell said: “It’s very convincing unless you know it’s untrue as I do and it was clearly aimed to destabilise me and finish me off by sending it into the heart of government to my deputy and could easily have done so very fast.”

In the midst of plebgate, Mitchell attracted widespread criticism for his silence on the issue, only denying the word 'pleb' was ever used. He didn't attend the Conservative Party Conference, insisting that his presence would merely distract from the party's agenda and prompt negative press.

He explained why he adopted this position, telling Channel 4: “Well when the story broke, the decision was made that I would apologise for what I did say and my apology was accepted, there was no police complaints and that we would let it lie. Now with the benefit of hindsight that was clearly the wrong decision.”

Former Tory leadership contender David Davis told the programme: “You try to avoid conspiracy theories in my business otherwise you’d go mad but it does look very weird, it does look very strange. How on earth could this have happened, this man wasn’t where he said he was, there wasn’t the crowd he said was there and his description reads like an extract from a police log itself. You have to say where did this come from?”